1. Field of the Invention
The present invention relates to the manufacture of a zinc-nickel alloy plated steel sheet having excellent corrosion resistance and used for automobiles, home electric appliances, building materials, etc. More specifically, the present invention relates to a zinc-nickel alloy plating solution for restraining defects on a surface caused in zinc-nickel plating and in particular needle-shaped stain on a plated surface and burning at the edge of a steel sheet.
2. Description of the Related Art
It is well-known that a zinc-nickel alloy plated steel sheet has a plated layer made to a single .gamma. phase when a nickel content is 10-16% and the steel sheet exhibits high corrosion resistance. As a method of industrially applying electrolytic zinc-nickel alloy plating to a steel sheet, there are known a method of using a sulfuric acid bath mainly containing zinc sulfate and nickel sulfate, and a method of using a chloride bath mainly containing zinc chloride and nickel chloride. Since it is difficult for nickel to be deposited in a plating layer in the sulfuric acid bath, an atomic percentage of nickel in a plating solution is larger than that of nickel in an alloy plated layer, and thus a lot of expensive nickel sulfate must be used in the plating solution. On the other hand, the chloride bath has an advantage that an atomic percentage of nickel in an alloy plated layer is approximately equal to that of nickel in a plating solution, and thus a plating operation can be easily managed.
When electrolytic zinc-nickel alloy plating is continuously applied to a steel sheet in the chloride bath, a white pattern called "needle-shaped stain" having a width of about 1 mm and a length of about 1-20 mm may be formed on a plated surface due to a fine irregular flow of a plating solution. Although the needle-shaped stain does not have any effect on corrosion resistivity, a commodity value of outside appearance of the plated steel sheet is lowered by it.
When a needle-shaped stain portion is observed, plated crystals become coarse granular crystals. A reason why such coarse granular crystals are made is that plating metal ions are insufficiently supplied due to the disturbance of the flow of the plating solution, and it is supposed that a portion containing the coarse crystals seems white.
On the other hand, when an electrolytic zinc-nickel alloy plating is continuously applied to a steel sheet by a continuous electroplating method using the chloride bath, since a plating solution in the chloride bath has a high electric transmission as compared with that in the sulfuric acid bath, a plating current is liable to concentrate on the edges of the steel sheet. When the current excessively concentrates, the diffusion and replenishment of ions cannot catch up with the concentration of the current causing a shortage of ions. As a result, the zinc-nickel alloy plating has a poor intimate contact property with a coarse grain size and a plated portion seems black. Such a black state of plating is usually called "burning".
Burning is liable to be produced in the electrolytic zinc-nickel alloy plating using the chloride bath due to the concentration of current on the edges of a steel sheet. When the burning occurs, burnt edges must be cut off by trimming because the portion does not have an economic value.
On the other hand, there have been examined methods of obtaining a plated steel sheet having an excellent outside appearance in the zinc-nickel alloy plating using the chloride bath. For example, there are a method of effecting plating at a flow rate of a solution of 20 m/min or higher disclosed in Japanese Patent Unexamined Publication No. 57-164999 (1982), and a method of adding at least one compound selected from the group consisting of aluminum chloride, ammonium aluminum chloride, ammonium chloride, potassium chloride, calcium chloride, barium chloride, sodium chloride, and magnesium chloride disclosed in Japanese Patent Unexamined Publication No. 58-55585 (1983). However, these methods intend to stabilize a plated composition by examining plating conditions and the composition of a precipitated alloy and do not solve the defects on a surface such as needle-shaped stain, burning and the like.
Further, although Japanese Patent Unexamined Publication No. 58-210189 (1983) discloses a method of using an organic brightener such as a .beta.-aminopropionic acid derivative, its copolymer etc., such a so-called brightener only improves the brightness of a plated surface and does not solve defects on a surface such as "needle-shaped stain", "burning" and the like. That is, a so-called brightener intends to obtain a leveling effect for smoothing an irregular state of entire plating and a brightening effect for making a plated surface bright by improving a reflectance ratio on the plated surface and does not restrain a partial unevenness such as "needle-shaped stain" and "burning" of edges, which is intended to be solved by the present invention. Further, there is not yet known a plating solution capable of solving the problem of "needle-shaped stain" and "burning" caused by the zinc-nickel alloy plating using the chloride bath.
Although a so-called brightener sometimes employs a surfactant and an organic compound as its main agent, an action of these components to plating is greatly different depending upon types of plating metals and plating solutions. Therefore, it is important to find types of components, combination thereof and a range of concentration thereof by which a target effect can be obtained in a specific plating solution for obtaining a specific plated metal, and the determination of these components, combination and range of concentration thereof is a novel technical achievement.